Magnetic material



P. P. CIGFFI MAGNETIC MATERIAL Apnl 16, 1929.

Filed Dec. 25, 1926 5 Sheelzs-Sheet l April 16, 1929. P mOFF-l 1,708,936

MAGNETIC MATERIAL Filed Dec. 23, 1926 5 Sheets-Sheet 2 H94 /f/g 5.

izpoa 2,000

lopoo lqoao apoo 8,000

aon-o epoo lepoo lzpoo lopoo lopao epoo aooo zpoo zgoo Wr/ley vApril 16, 1929. P, P lOl-Fl 1,708,936

MAGNETIC MATERIAL A Filed Deo. 23, 1926 5 Sheets-Sheet 3 lqooo spoo MAGNETIC MATERIAL Filed Dec. 23, 1926 5 Sheets-Sheet '4 Y am ` f'ar/rey April 16, 1929. p P, lOl-'Fl 1,708,936

MAGNETI C MATERIAL Filed Dec. 25, 1926 5 Sheets-Sheet 5 nooo-- Pay/ c/bff/ Patented Apr. 16,; 1929.

lPAUL r. cIoFFI, or BROOKLYN,

TOBIES,

INCORPORATED, or NEW groan,

NEW YORK, .ASSIGNOR Tb BELL TELEPHONE LABORA- l'. Y., A. CQRPOBATION 0F NEW YORK.

MaGNE'rIc'- MATERIAL.

,application mea nenember'e's, me. serial Nn. 156,615.

This invention relates to magnetic materials and-particularly to a process for devel-Y oping desired materials.

An object of the inventionvis to improve the magnetic properties of materials.

magnetic properties in such A specific object is to obtain substantially i 'constant permeability in a magnetic material over a wide range of magnetizing forces.-

samples of the same material were subjected to the same heat treatment, however, the magnetization curve was' non-linear. 'It is .believed as the result of extensive experiments that this difference in the magnetic characteristics. of the material is due to the effect on the material of extraneous magnetic fields, suchas that of the earth. In accordance with the present invention in a broad aspect, a magnetic materialin which a substantially linear relation, between magnetizing Aforce -vand magnetization exists and which, thereore, possesses a substantially constant permeability over awide range of magnetizing forces and other desirable properties, is obtained by heating .the material and subsequently cooling it in ,4o a lspace in which there is substantially no field tending to magnetize: it in magnetic the direction of the magneticaxis which the material will have in use. l with a specific'embodiment of the invention,

v4.5 a method is provlded for obtaining constant permeability and other desirable properties in a magnetic material consisting in applying magnetizing forces ,for counteracting the magnetizing effect .of extraneous fields acting on the material during the heat treatment. For example, the material while being heat treated may be subjected to an applied field which is equal and opposite to t field vtending e component of thevearths to magnetlze the material.

to which the nickel-iron-cobalt a oy. i sponding to the curves of Fig. 2.

straight specimens o ,from the In accordance.v

The characteristics 'of certain magnetic materials, other than constant permeability, obtainedl b substantially neutralizing the magnetlc' elds acting upon the material durlngl heat treatment, are negligible hysteresls, remanence and coercivity. A articular magnetic material to whic this mvention is applicable'.:consists of approxiv mately 45% mckel, 25% cobalt and30% iron with about 0.5% manganese added tov increase the workability of the materiali The curves given in Figs. 2 to 14 inclusive relate to specimens of this particular coml, position but are illustrative ofthe roperties of other magneticmaterials wit n the 70 scope of this invention. in-all of the igures of the drawing involving magnetizing forces and flux dens1ties,'these. quantities are plotted .in c. g. s. units.-

The invention may be readily understood by referring to the lfollowingl description i and the drawing in whichz` y Y Fig. 1 is a schematic view of an apparatus which may be employed for subjecting a magnetic-material to heat treatment in accordance iviththisl invention.

Fig.a shows magnetization curves'or straight and fiat iraled specimens of a Fig. 3' shows .permeability curves corre- S5L 4 shows ma netization curves for the allo which were heat treated orvarious perio s of time.

Fig. 5 shows -ma etlza'tion curves for straight specimens o the alloy which were heat treated at various temperatures.

The curves of Fig. 6 show the effect on magnetization of slowly cooling the`Y alloy maximum temperature employed in the heat treatment.

Fig-7 shows magnetization curves fora straight specimen of the alloy which lwas subjected to lalternate rapid and slowcool- Fig.

'The curves of Fig. 8 show the eiect on permeability. of reducing the cross section of 'a straight specimen of the alloy by etching.' 1 l L j Fig. 9 shows magnetization. curves for 105 straight specimens of the alloy which. were cooled from the heat treating temperature at widely-different rates.

Fig. 1o indicates the erect m the mag.'

netizaticn curves for a straight specimen of the alloy of adding a circular field 'to the field acting longitudinally in the' material during the hea-t treatment.

Figgmllshows magnetization curves for straight specimens of the alloy which were heat treated in magnetizing fields of different intensity, respectively.

Fig. 12 shows permeability curves correponding to the magnetization curves of lig. 1l.

And Figs. 13 and 14 show hysteresis loops for two of the specimens for which magnetizatioin and permeability curves, respectively, are shown in Figs. 11 and 12.

Fig. 1 of the drawing shows an apparatus which may be employed for heat treating a magnetic material inv accordance with this invention. The glass cylinder 20 is closed at one end and is provided at the other end with an air tight stopper 21 through which a metallic tube 22 passes. This tube is connected to a pump, notl shown, for exhausting the air from the cylinder 20. One end of a specimen of magnetic material 23 which is to be heat treated is suitably attached to the metallic tube 22 at one end and a suitable weight 24 of conducting material is secured t9 the `other end. This weight is electrically connected through the mercury column 26'with an electrode 25, which is sealed into the glass tube 20. A source of alternating current 35 is employed for heating the specimen of magnetic material 23.

The magnitude of this heating current may be varied by means of the variable resistance 28 and measured by the meter 27. A wind ing 29 positioned about the cylinder 20 is employed to set up a magnetic field about the magnetic material 23 for substantially neutralizing the effects of other fields acting therein. The battery 30 supplies current to the winding 29, this current being varied by a variable resistance 31 and measured by a meter 32.

The curves of Figs. 2 and 3 indicate the difference in magnetic characteristics between two specimens of nickel-iron-cobalt alloy, one of which was in the form of `a tape wound in a fiat spiral and the other of which was in the form of a straight strip. This tape was 0.126 wide and 0.006 thick and the fiat spiral specimens consisted of about 50 turns of this tape forming a loosely wound toroi'd 2.84 inside diameter and 3.54 outside diameter.

The heat treatments to which the specimens employed in obtaining the curves of Figs. 2 and 3 were subjected, were nearly identical and consisted in heating specimens in an electric furnace at a temperature of about 1100o C. for approximately one hour and then cooling at the rate of about 200 C. per hour. It was found that the fiat spiraled specimen (curve a of Figs. 2 and 3) had a substantially constant ratlo of flux I gauss.

.7800 C., 710 C. and 6400 ,two minutes after which the temperature was lowed by rapid cooling,

density (B) to field intensity and therefore a practically constant permeability for values of. field strength up to nearly 3.0 "'On the other hand, in the case 0f the straight specimen (curve b of Figs. 2 and 3), it is indicated that the permeability increases rapidly with field intensity. The specimens to which the remainder of the curves relate (except curve a of Fig. 10) were heat treated by suspending them ver-r tically in a long cylinder, such as shown in Fig. 1, and passing a current therethrough. Ip this process sudden cooling Ais accomplished by opening the electrical circuit supplying the current while slow cooling is brought about by slowly decreasing the heating current. The temperature of the strip was measured by means of an optical pyrometer.

Fig. 4 shows the results of heat treating straight strips oi the alloy at 1000O C; and 710 C. for various periods of time. The cur. relating to the specimens heat treated at 1000o C. -for 2, 6 and 20- minutes are designated respectively by a, and c 'of' this ligure and the curves relating to the specimens heat treated at 71000. for the same periods are respectively resignated by d, e and f. The curves show that for the same heat treating temperature there are practically no differences in initial magnetization with respect to the time of heat treatment and only'small differences are indicated for higher intensities of magnetization.

The curves of Fig. 5 indicate the effect of temperature of heat treatment on magnetic behavior. curves designated by the letters a to t, respectively, relate, were maintained at 1060o C., i000o C., 970o C., 920o C., 850 C., C. for a period of The specimens, to which thel rapidly reduced to normal by opening the current supply circuit. These curves indicate that the magnetization goes on in two steps: Van initially slow and almost linear change in fiux with magnetizing force, followed by a rapid rise of flux for further small ch`aI'1ges` in magnetizingforce. The initial permeability is low and ,is practically independent of the temperature of heat treatment as is indicated by the common portion ofthe curves in the initial stages of magnetization. The effect of increase in temperature is simply to shift the point of divergence from the common curve nearer to the axis of the ordinates. Thus, for a short time of heating by means of an electric current, fola suggestion of the presence of linear magnetization with respect to magnetizing force is obtained, the region of linearity being smaller the higher the temperature of heat treatment. Y

The-specimen to-which curve a of Fig. 6

relates 'wasmaintained' at a temperature of 10000 C. for two minutes and then cooled to ture of 10000 C. for two minutes and rapidroom temperature in 14 minutes. The speci' men to which curve b relates-was-heated to 7100 C. for two minutes and also cooled to room vtemperature in fourteen minutes. The effect of slow cooling,` asindicated by these curves? is to increase the range of field intensity over which the magnetizationmremains linear, this range being 0.4 gauss for the higher temperatureof anneal and 0.7 gauss for the lower temperature. Although the initial permeability of the strip is much higher than that obtained with the flat s iraled specimen, the range over which tie linear relation holds'between magnetization and magnetizing force is considerably lower.

The curves of Fig. 7 are for a specimen which was subjected to alternate rapid' and slow cooling from the same temperature. This specimen was first heated to a temperacooled. Curve w was then obtained, after which the specimen was again maintained at 10000 C. ,for two minutes and cooled to normal temperature in fourteen minutes. After obtalning curve b the specimen was 'again' raised to a temperature of 10000 C., where it was maintained for two lminutes, and en rapidly cooled. The data for curve: c was then taken. The curves indicate that heating and slo'wvcooling has a tendency to nullify the effect of heating and rapid cooling andl vice versa. That the nulligfying e'e'ct is not complete iso indicated, by the/slight vdifference existing' between curves a and c of Fig. 7 and between curve b of Fig. 7 and v curve b of Fig. 6.

In order to determineif the effect of the `cooling rateon the'shape -ofthe magnetization curve for the materlal was due to strains set up therein a specimen was heated, then cooled rapidly and tested, after which the. surface layer of the specimen was removed by etching and the test repeated. If the magnetic behavior of the strip is affected by strains set up on the surface due to the rapid cooling, then 'the removal of 'the surface layer b etching-should alterthe magnetization. urve a ofFig. 8 shows the magnetic' characteristics of a straight specimen` which was heated at 9700 C. for twominutes and then rapidly cooled. The strip was then reducedzin thickness from 0.006 to 0.003"

v by etching after wI ich dui-ve b was obtained.

The data from which curve 'is plotted wasA .taken after a further reduction lin thickness rlhese-curves .indicate that thev magnetic behavior ofthe .strip is not appie' ciably affected by the removal layer of the material. Fig. 9 shows the similarity in Amagnetic of the surface behavior that isA obtained when employing wide differences in cooling rates. The specimens for which the curves of Fig. 9 were obtained Vwere heattreated by. being subjected to a temperature'of 10000 C. and then slowly cooled. Curve @of thisgure relates toa specimen whichwas maintained at this temperature for s ix minutes' and .cooled to normal temperature in fourteen minutes while the specimen to which curve b relates was raised to the same temperature where it was also maintained for six minutes and then cooled to normal .temperature in 8.5 hours.

dicate that some cause other than strain in the material contributesgtoward the magnetic behavior of slowly cooled electrically heated strips.

The specimens with which the curves of Fig. were obtained were heat treated by being subjected to a temperature of 11000 C. and then slowly cooled. The strip to which curve a relates was annealed in an electric furnace while the other strip to which curve b-relates was heated by passing alternating current through the strip. It is believed that the dierence in magnetic characteristic f the two specimens is due to the fact that theformer was subjected, while being -,heat treated; to a component of the earths field acting longitudinally in the strip and the latter was subiected both to this field and to a circular field produced by the heating curp5 y rent ilowing; through thestrip. It appears lthat the atomic orientation obtained by the latter kind of magnetization at or near the` .Curie point would be sufliciently different v brought about by the two methods of heat to account for the difference in behavior 47K5 These curves and the curves of Fig. 8 in field substantially equal and opposite tothe I vertical-component of the earths field, thus neutralizing the magnetizing effect of the earth-s field on the material. This curve indicates a linear relation between field intensity and flux density and, therefore, a constant value cfpermeability for ux densities below 2000. Curve b is for a sample of the material which was heat treated while under the influence of the vertical component of the earths field acting longitudinally in the material, that is, the neutralizing feldwas not applied to `thissample. A departure '125 from the linear characteristics of curye a' is indicated, there being an abrupt rise influx density from about 300 to 1000 c. g. s. units y for a change in field intensity from 0.4 to 0.5 auss. Curve c .shows therelation between ux density and fieldv intensity for a sample of the material which was heat treated While being subjected to a magnetizing field twice as great as that of the vertical component of the earths field acting in the longitudinal direction in the material, that is, the earths field was supplemented by an equal artificially applied field. This curve indicates an abrupt'rise in flux density at a value of field 'intensity of about 0.2 gauss. Curve d shows the results obtained with a sample which was heat treated With its longitudinal axis in line with the horizontal component of the earths field. This curve rises from zero toga fiux density of approximately 12,- 000 e. g. s. units for a change in field intensity from 0 to 0.1 gauss and then abruptly decreases in slope, rising from 12,000 to 14,000 c. g. s. units in flux density for a change in field intensity from 0.1 to 1.0 gauss.

Curves a, Z1, c, and Z of Fig 12 are permeability curvesfor specimens Whose magnetization curves are designated' as b, c and d, respectively, of Fig. 11.

vCurves a and d of Fig. 13 and 14 are the hysteresis loops for the specimens Whose magnetization and permeability curves are shown in curves a and al, respectively of Figs. ll and 12. .It is indicated by these curves that the effect of heat treating the specimen in a substantially neutral magnetizing field is to considerably decrease the hysteresis, remanence, and coercive force of the material. Curve a of Fig. 13 is a substantially straight line passing through the origin, a

fiux density of about 500 c. g. s. units being employed. When a higher flux density of approximately 1500 c. g.' s. units is employed a small hysteresis loop is obtained as shown in curve a; of Fig. 14.

` In comparing the various curves, We find l that a magnet-ic material characterized by a linear relation between field intensity and fiux, density, that is to say, by a constant value of.permeability over a wide range of magnetizing forces is obtained by subjecting the material to a heat treatment While at the sametime preventing magnetization of the material. Other magnetic properties simultaneously developed by this treatment are negligible remanance, negligible coercive force and the absence of hysteresis. One method of preventing the magnetization of the material is to subiect it to an artificially applied magnetizing field which is equal and opposite to any field acting Within the material tending to magnetize it. However, otherl methods of preventing the magnetization of the material during the heat treatment may be employed as, for example, shielding the material from extraneous fields by surrounding it with a magnetic enclosure.

It has been found, moreover,y that cross permeability and other magnetic properties v mentioned above byA heat-inw the magnetic material in -a space in whic there is substantially no magnetizing field tending to magnetize it in the direction of the magnetic axis the material Will have in use.

What is claimed is- 1. The method iof obtainin substantially constant permeability in a nic el-iron-cobalt alloy over a Wide ran'ge of field intensities, which consists in heating the alloy and subjecting it While cooling to a field for substantially neutralizing the magnetizing effect of extraneous fields acting on the alloy.

2. The method of increasing the constancy of permeability in a substantially straight sample of magnetic material which consists in suspending the material in a substantially vertical direction, passing a heating current through said material, subsequently gradually reducing said heating current to zero and simultaneously subjecting said material to a magnetic field equal and opposite to the Vertical component of the earths field.

3. The method of obtaining in a magnetic alloy composed of approximately 45% nickel, 25% cobalt and 30% iron, substantially constant permeability over a Wide range of magnetizing forces, which consists in heat treating the alloy and at the sametime applying a magnetic field thereto ,for reducing the magnetizing effect of other fields acting on the alloy.

- 4. The method of improving the magnetic properties of an alloy containing as essential constituents, iron, nickel and cobalt, which consists in heating the alloy and subjecting it to the action of an artificially a plied magnetic field While it yis being coo ed.

5. The method of reducing any or all of the coercive force, remanent magnetism, or hysteresis loss, in a magnetic material including as essential constituents, iron, nickel and cobalt, which comprises heating the ma.

PAULVP. oIoFFI. 

